Liquid scintillation counters are mainly used for measuring low energy beta radiation emitting samples, which are of, for example, biological or medical interest.
The range of the low energy beta particles in the sample is generally few tens of micrometers at the most. As a consequence, the sample to be measured has to be placed in direct contact with the scintillation medium, which comprises a solvent or solvents and a solute or solutes present in a small portion by weight of the solutions. In this interaction process most of the kinetic energy of the interacted beta particle is absorbed by the solvent and then transferred to the solute which emits scintillation photons, whose amount is proportional to the energy of the interacted beta particle. These scintillation photons are detected usually by two, in coincidence operating, photomultiplier tubes that produce electric pulses. The heights of the pulses are proportional to the amount of emitted scintillation photons and thus proportional to the energy of the interacted beta particle.
Traditional liquid scintillation counters are designed to count samples--one at a time--deposited with scintillation liquid into sample vials. The volume of the sample vial is typically 6 or 20 ml. The sample vials are deposited into vial racks, which have separate compartments for individual sample vials. The sample racks are placed on the conveyor of the automatic sample changer system of the counter.
Because the above mentioned liquid scintillation counter is designed to count vials, whose volume is up to 20 ml, serious difficulties are encountered, when the sample volume is only few hundred microliters or less. Typically, these samples are prepared in minivials which are then inserted into normal vials. In addition, the handling of separate sample vials is very time consuming and includes potential risks of mis-identification. The sample changing mechanism of such an instrument is also rather complicated, because the vial must be removed from the sample rack and must be positioned into a light tight radiation detection chamber, and after counting it must be returned to the same position in the sample rack.
A novel liquid scintillation counter, which counts samples directly from multi-well sample plates is shown in U.S. Pat. No. 5,061,853 (Lehtinen et al), which apparatus counts liquid scintillation or corresponding samples directly from sample plates which comprises several separate sample wells or vials. The apparatus has one or several detectors in order to count one or several samples at a time. The sample plate is placed, in the counting position or before counting position, manually or automatically on a rigid plate holder made of photon attenuating material and having holes for the wells of the sample plate. As a consequence, an optically isolated compartment is formed around each sample well of the sample plate. The walls of the holes are reflecting or scattering in order to guide the photons from the liquid scintillation sample to the detectors, built of two photomultiplier tubes operating in coincidence and situated on the opposite sides of the holes of the plate holder. The wells of the sample plate can be closed by an adhesive transparent tape. The apparatus can be used also for counting gamma radiation emitting samples if the holes of the sample plate are surrounded by gamma radiation sensitive detectors. A detailed construction of this kind of liquid scintillation counter is shown in PCT patent application no. PCT/F190/00124 (Sonne et al.).
Another novel liquid scintillation counter, which counts samples directly from multi-well sample plates is shown in U.S. Pat. No. 5,039,860 (Yrjonen et al.), which is an apparatus for counting liquid scintillation samples and which includes a sample plate containing a plurality of cup-shaped sample wells, a photomultiplier assembly over the tops of said cup-shaped sample wells, a photomultiplier assembly beneath the bottoms of said cup-shaped sample wells and light reflective tubes extending upwardly from said lower photomultiplier assembly and surrounding each cup-shaped sample well. In order to prevent light from travelling between the sample wells inside the material of the sample plate, one or both of the surfaces between the wells of the sample plate are painted black or this region is treated in some other way which prevents light from travelling between the sample wells inside the material of the sample plate.
Third novel scintillation counting system for in-situ measurement of radioactive samples in a multiple-well plate is presented under European Patent Publication Number 0425767A1 (VanCauter et al.). This apparatus is provided with multiple photomultiplier tubes positioned adjacent to the sample wells containing the scintillator for simultaneously measuring the radioactivity of multiple samples with only a single photomultiplier tube sensing the scintillations from each well and converting the sensed scintillations into corresponding electrical pulses. The electrical pulses from each photomultiplier tube are processed to discriminate between pulses attributable to sample events within the wells and pulses attributable to non-sample events such as photomultiplier tube noise. The discrimination is effected by determining whether a selected number of electrical pulses occurs with a prescribed time interval, the occurrence of the selected number of pulses within the prescribed time interval signifying a sample event. Only the electrical pulses attributable to sample events are supplied to a pulse analyzer.
U.S. Pat. No. 4,933,554 (Lehtinen et al.) describes a method of producing a carrier for a plurality of radioactive samples to be monitored in a liquid scintillation counter, where wells are provided in a rigid plate of a photon attenuating material in that holes disposed in a matrix configuration are covered from one side of the plate by a first photon permeable foil, sample carrying cut-out from sorption sheet on which samples have been deposited are placed in said wells, a scintillation fluid is added to the wells, and the wells are sealed by a second permeable foil to keep the cut-outs and the scintillation fluid within the respective wells.
A commercial product of a multi-well sample plate which is provided with light impermeable well walls but light permeable bottoms is manufactured by Polyfiltronics Ltd., UK. In this patent application this kind of plate is called as a bottom-window multi-well sample plate.